Merocyanine 540 as a fluorescent probe of membranes: staining of electrically excitable cells.

With the exception of certain blood cells considered in the accompanying paper (Valinsky, Easton and Reich, 1978), merocyanine 540 (MC 540), a fluorescent membrane probe, selectively strains the membranes of a wide variety of electrically excitable cells, but not those of nonexcitable cells. This reaction is Ca2+-dependent when staining is performed in buffered iso-osmotic sucrose, Ca2+-independent when staining proceeds at high ionic strength, inhibited by La3+ and sodium Suramin, enhanced by controlled, low level photosensitization of cell-associated dye and essentially irreversible. These characteristics of the staining reaction depend upon the maintenance of both cell viability and a normal unperturbed membrane structure. Although the mechanisms involved in the staining specificity remain unknown, observation of MC 540 partitioning between benzene and water in model reactions indicates that dye transport into hydrophobic solvents is accompanied by the formation of stoichiometric complexes with cations and phospholipids. These results may suggest the existence of specific, possibly phospholipid-rich membrane domains that mediate complex formation with MC 540 in excitable cells; comparable domains either would not exist, or would be inaccessible at the external surfaces of nonexcitable cells.